A cross section of a conventional permanent magnet DC motor is shown in FIG. 1. Such motors use a cylindrical shell 1. A rotor 3 is received in the cylindrical shell 1. Permanent magnets 2 are fixed to the inner surface of the cylindrical shell 1 and are distributed around the rotor 3. However, the motor with such outline structure has the following problems: 1) it is easy to roll in the process of transport; 2) the installation is inconvenient and an additional positioning device is required.
A square motor developed in this field in recent years is shown in FIG. 2. The motor uses a square shell 1′. Permanent magnets 2′ are installed at the inner surface of the four sidewalls of the shell 1′. A rotor 3′ is received in shell 1′. The Permanent magnets 2′ are distributed around the rotor 3′. Although such motor with square shell solves the above-mentioned shortcomings of conventional cylindrical motor, it has the following problems: 1) the shell has low space utilization rate, and the space of four angles are not used; 2) the diameter of rotor is equal to the size of shell minus the thickness of shell and the thickness of the permanent magnet. In the requirement of the motor shell having relatively small size, the diameter of rotor 3′ is restricted by the thickness of the shell 1′ and the magnet 2′.